The exhaust valve to be used to Diesel engine is easily burnt by exhausted gas, and this is remarkable in middle or high speed Diesel engine issuing the gas of high temperatures, especially in a case of using inferior or bad oil. Such problem involved with burning is in general found in blowing at a valve body and a valve seat composing the exhaust valve. The exhausted gas of Diesel engine much contains, in relation with the fuel, oxides of low melting point as V.sub.2 O.sub.5 or Na.sub.2 SO.sub.4, and these oxides penetrate into the seat and cause oxidization accelerated at high temperatures so that said blowing and burning occur. The prior art has employed Cr-heat resisting steel or Ni-based super heat resisting alloy for the mother material of the valve body and the valve seat in order to provide countermeasures to avoid said phenomena. A portion to compose the seat of the mother material is prepared with weld padding or coat padding of corrosion resistible alloy of Co based or Ni based high hardness (Hv 600 to 700). However when the fuel is inferior, the seat would be instantly hurt by blowing and burning, since it is only padded with the corrosion resistible alloy. On the other hand, there has been an attempt for coating on the mother material a substance where ceramics is dispersed in Co based or Ni based alloy, but such coated layer of metals and ceramics uniformly dispersed is poor in durability against repeated shocks. In addition, the coated layer dispersed with ceramics is low in density, and the compound of low melting point which accelerates oxidization at the high temperatures penetrates into the coated layer and further to the mother material, so that the blowing-burning is invited in turn. It may be also assumed to form the seat with ceramic layer for assuring corrosion resistibility, but since such seat is poor in thermal shock resistibility and toughness, cracks or exfoliation are easily effected and its practicability is very difficult.
The present invention is to remove defects as mentioned of the exhaust valve for Diesel engine and is to offer improvement of this kind of exhaust valve.
An object of the invention is to impart excellent corrosion resistibility, thermal shock resistibility and toughness to the seat, and offer such an exhaust valve which exactly avoids the blowing-burning or exfoliation.
Another object of the invention is to offer such an exhaust valve which is imparted with hard property to the seat, thereby to avoid damages by invasion of hard substances in the seat.
Another object of the invention is to offer such an exhaust valve which provides satisfactory adhesion between the seat containing ceramics and the mother material.
Another object of the invention is to offer such an exhaust valve which exactly avoids adhesion of burnt harmful remainders to the seat.
Another object of the invention is to offer such an exhaust valve which is imparted with heat insularity to the seat.
Another object of the invention is to offer such an exhaust valve which is imparted with more excellent corrosion resistibility, thermal shock resistibility, toughness and adhesion with the mother material, by making structure of the seat closer.
A further object of the invention is to offer structure of the seat for obtaining said excellent characteristics, and materials suitable for obtaining said structure.
A still further object of the invention is to offer a method of making an exhaust valve having the above mentioned various characteristics industrially and most efficiently.